The Pattern of Change in U.S. Streamflow Harry F. Lins U.S. Geological Survey Climate Change and Water Resources USACE-USGS Joint Meeting 31 May 2007

Warmer temperatures will lead to a more vigorous hydrological cycle and . . . increased frequency of extreme rainfall events, increased frequency and magnitude of flooding, declining low streamflows. Source: Intergovernmental Panel on Climate Change (IPCC) Assessments, 1995, 2001, 2007

Hydro-Climatic Data Network (HCDN) >1,500 streamgages >75,000 water years of daily mean Q >400 gages with 60 years of record to present Source: Slack and Landwehr, 1992, USGS OFR 92-129

Trends in U.S. Streamflow, 1940-1999 435 Stations; p ≤ 0.05 Source: Lins and Slack (2002) GRL; (2005) Phys. Geog.

Source: Lins and Slack (2005) Phys. Geog. Regional Pattern of Trends Percent of Stations Having Trends, by Water Resources Region, 1944-2003 QMin QMed QMax Note: Blank cell indicates less than 10 percent of stations had trends. Source: Lins and Slack (2005) Phys. Geog.

Shifts in Timing of Qmax and Qmin by decade, 1940-1999 Monthly Variation in the Number of Annual Maximum (top) Minimum (bottom) Discharges by Decade, 1940-99 Explanation Mean of period (horizontal line) 1940-49 (left-most vertical bar) 1950-59 1960-69 1970-79 1980-89 1990-99 (right-most vertical bar) Source: Lins and Slack (2005) Phys. Geog.

Seasonal Subseries Plot 1940-49 1950-59 1960-69 1970-79 1980-89 1990-99 1940-99 Qmax

Seasonal Subseries Plot Qmax

Shifts in Timing of Qmax and Qmin by decade, 1940-1999 Monthly Variation in the Number of Annual Maximum (top) Minimum (bottom) Discharges by Decade, 1940-99 Explanation Mean of period (horizontal line) 1940-49 (left-most vertical bar) 1950-59 1960-69 1970-79 1980-89 1990-99 (right-most vertical bar) Source: Lins and Slack (2005) Phys. Geog.

Regional Flow Seasonality and Monthly-Scale Timing Shifts Source: Lins and Slack (2005) Phys. Geog.

Nationwide StreamflowTrends Mean Standardized Departures for 400 Stations, 1941-99 Source: McCabe and Wolock (2002) GRL

Trends by Beginning and Ending Dates for 400 Stations, 1941-99 Annual Minimum Annual Median Annual Maximum Source: McCabe and Wolock (2002) GRL

Are Floods Increasing?  Data in USGS Peak Flow File for 23,526 sites  Of these, 2,515 had NWS identified flood stage  Of these, 825 had complete records for 1941-2000  Of these, 247 did not have qualification codes associated with stage datum, backwater, dam failure, regulation, diversion, ice-jam, urbanization, mining, agricultural changes, or channelization  Of these, 200 were unregulated, 47 regulated

Percent of Streamgages with Annual Peak Above Flood Stage, 1941-2000 247 streamgages: 200 unregulated, 47 regulated

Average Flood Flow, 1941-2000 247 streamgages: 200 unregulated, 47 regulated

Summary of Observed Streamflow Trends  Streamflow increased in the U.S. between 1940 and 1999 40-45% of streamgages had increases in Qmin to Q60 flow while 10% had increases in Q100 flow  Increases occurred as a step-trend around 1970  Increases observed in nearly all regions of U.S., except Upper Colorado, Great Basin, PNW, and California  No monthly-scale shifts in the timing of Qmin, Qmed , and Qmax  No trend observed in frequency of floods, although an increase was observed in the volume of flood flows at unregulated sites